Optical tandem photovoltaic cell panels

ABSTRACT

A solar energy conversion device comprises a vertical stack of at least two panels stacked in a hierarchy from an upper panel to a lower panel with each of the panels including a matching array of solar cells having a different energy bandgap from other panels of solar cells in the vertical stack of panels. Each panel in the vertical stack may be arranged with one of the panels having solar cells with a higher energy bandgap situated in the hierarchy and in the stack above others of the panels containing solar cells with a lower energy bandgap. The top surface of the device is adapted for receiving solar energy incident upon the uppermost panel. Each upper panel absorbs a fraction of sunlight with larger solar photon energies larger than the energy bandgap thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/242,962 filed Oct. 1, 2008 entitled “Optical Tandem Photovoltaic CellPanels,” the disclosure of which is incorporated by reference herein.

This application contains subject matter which is related to the subjectmatter of the following commonly assigned, copending applications,including U.S. patent application Ser. No. 12/243,995 filed Oct. 2, 2008of Hovel entitled “Quantum Well GaP/Si Tandem Photovoltaic Cells”, nowU.S. Pat. No. 8,101,856, and U.S. patent application Ser. No. 12/246,511filed Oct. 7, 2008 of Hovel entitled “Tandem Nanofilm Photovoltaic CellsJoined by Wafer Bonding.” Each of the above listed applications ishereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

This invention relates to photovoltaic types of Energy ConversionDevices (ECDs), and more particularly to the photovoltaic cell, e.g.cell, type of ECD, which consists of a stack of panels with an array ofphotovoltaic cells mounted thereon.

DEFINITIONS

Electromagnetic Radiation to Electric Energy Conversion Device (EREECD):A device that reacts with electromagnetic (optical) radiation to produceelectrical energy

Optical Radiation to Electric Energy Conversion Device (OREECD): Adevice that reacts with optical electromagnetic radiation to produceelectrical energy. Such a device could be a radiation absorbing device,e.g. a photodetector/counter, photovoltaic cell (solar cell) or aradiation-driven electrolysis cell.

Optoelectronic Energy Device (OED): A device that reacts with opticalradiation to produce electrical energy with an electronic device.

Photovoltaic cell: An electrical device (e.g. a semiconductor) thatconverts light or other radiant energy, in the range from ultraviolet toinfrared radiation, incident on its surface into electrical energy inthe form of power/voltage/current which has two electrodes, usually adiode with a top electrode and a bottom electrode with oppositeelectrical polarities. The photovoltaic cell produces direct currentwhich flows through the electrodes. As employed herein, the termphotovoltaic cell is generic to cells which convert radiant energy intoelectrical energy including EREECDs, OREECDs, and OEDs as defined above.

Solar cell: An electrical photovoltaic device (e.g. a semiconductor)that converts light incident on its surface into electrical energy whichhas two electrodes, usually a diode with a top electrode and a bottomelectrode with opposite electrical polarities. The solar cell producesdirect current which flows through the electrodes. As employed herein,the term solar cell is generic to cells which convert radiant energyinto electrical energy.

Panel: A structure formed on a substrate formed of glass, quartz, metal,or other material with multiple solar cells mounted thereon which mayinclude means of electrical input and output.

TCO (Transparent Conducting Oxide): an optically transparent electricalconductor.

Stack: A cascade of panels mounted on top of each other arranged so thatsunlight is incident on the uppermost panel and a portion thereoffilters down or passes through to the underlying panels.

Solar spectrum: The amount of solar energy available as a function ofthe wavelength of the incident photons, where the energy of each photonis 1.24 volts divided by the wavelength in microns.

Spectral response: The amount of electrical current produced by a solarcell at the wavelength of an incident photon.

Electrically parallel: Solar cells with a connection of the topelectrodes and bottom electrodes with the top electrodes and bottomelectrodes of other solar cells producing additive solar cell currents.

Electrically series: The connection of the top electrode of a solar cellto the bottom electrode of another solar cell so that the voltagesproduced by the cells add together.

Optically series: An arrangement whereby light incident on one device,e.g. a semiconductor, is partly absorbed in that device and theremainder is passed down to other devices (e.g. semiconductors.)

Tandem solar cells: A stack of solar cells in which some incident lightis absorbed by higher-lying cells and the portion not absorbed is passeddown to lower-lying cells.

Bandgap or Energy Bandgap: The characteristic energy profile of asemiconductor device that determines its electrical performance, currentand voltage output, which specifically comprises the difference betweenthe valence band and conduction band of the semiconductor.

p/n junction: A diode formed by the connection between a p-type and ann-type semiconductor.

Tunnel junction: A p/n junction doped so highly as to exhibit ohmicelectrical behavior, rather than diode behavior.

Contact grids: Metal lines connected together to gather currentgenerated by a solar cell with low electrical resistance and to allowincident sunlight to reach most of the surface of the solar cell whichis a semiconductor device.

Load: A device using power, e.g. an appliance, heater, television, etc.requiring a supply of power.

Base: The main body of a solar cell, lying below the junction boundaryin a semiconductor.

By-pass diode: A diode that is placed across a group of solar cells toshunt excess current and therefore prevent damage to cells which becomeshaded from light while others in the group remain illuminated.

The purpose of fabricating tandem solar energy conversion devices usingmultiple kinds of solar cells with different bandgaps is that solarcells are more efficient in converting photon energies close to theirbandgap than they are at converting photon energies much higher thantheir bandgap. By subdividing the solar spectrum into portions and usingmultiple solar cells optimized to convert solar energy into electricalenergy within an appropriate portion of the spectrum, the overallefficiency is increased considerably.

Certain types of tandem solar cells are well known in the art. The mostcommon variety is the monolithic form in which a p/n junction is formedin a first semiconductor followed by a tunnel junction grown by epitaxy,a second junction of a higher bandgap semiconductor also grown byepitaxy. If desired, a second tunnel junction and a third p/n junctionof an even higher bandgap semiconductor are also grown by epitaxy. Eachsemiconductor p/n junction located above other semiconductor p/njunctions in a tandem stack thereof absorbs the light of photon energylarger than its bandgap and transmits the remainder of the light ofphoton energy below that bandgap to the semiconductor p/n junctionslying therebelow. The purpose of the tunnel junctions is to act as lowresistance “ohmic” contacts to connect the separate p/n junctions inseries, causing the voltages of each cell to add to the others. Sincethe junctions are in series, the currents generated by each p/n junctionsolar cell in the stack must be the same or power and energy conversionefficiency will be lost.

SUMMARY OF THE INVENTION

None of the prior art includes the steps of connecting the solar cellson separate panels of a tandem structure so that each panel in the stackhas substantially the same voltage or current output, allowing thepanels to be connected in series to add voltages or parallel to addcurrents, whereby the finished tandem energy conversion device has onlytwo electrodes (one output electrode usually connected to electrical“ground” and one other output electrode) so that the energy conversiondevice can be connected to a single load.

In accordance with this invention a photovoltaic device comprises stacksof photovoltaic cells, e.g. photovoltaic (e.g. solar) Energy ConversionDevice (ECD) cells, with each stack consisting of a plurality ofphotovoltaic panels with an array of photovoltaic cells, e.g. solarcells, mounted on each thereof so that when radiant (solar) energy isincident on an uppermost panel a portion of the radiant (solar) energyspectrum is absorbed thereby producing electrical energy. Eachsuccessive ones of the stacked panels receives portions of the radiant(solar) energy spectrum not absorbed by an upper panel. The energy whichis transmitted to a panel of photovoltaic cells lower down in the stackis absorbed thereby producing electrical energy.

In one aspect of this invention, panels higher in the stack are mountedwith photovoltaic (solar) cells having higher energy bandgaps andsubsequent panels lower down in the stack are mounted with photovoltaic(solar) cells having lower energy bandgaps. The photovoltaic (solar)cells are connected in electrical series and parallel such that eachpanel produces a desired voltage and current output and the panels areelectrically connected together such that the amount of the incidentsolar power converted to useful electricity is greater than would beproduced by any one panel alone.

In accordance with this invention, a photovoltaic (solar) energyconversion device is provided including a vertical stack of at least twopanels stacked in a hierarchy from an upper panel to a lower panel witheach of the panels including a matching array of photovoltaic (solar)cells having a different energy bandgap from other panels ofphotovoltaic (solar) cells in the vertical stack of panels. Each of thepanels in the vertical stack is arranged with one of the panels havingphotovoltaic (solar) cells with a higher energy bandgap situated in thehierarchy and in the stack above others of the panels containingphotovoltaic (solar) cells with a lower energy bandgap. A top surface ofthe device is adapted to receive radiant (solar) energy incident uponthe uppermost panel.

Each of the upper panels absorbs a fraction of sunlight with largersolar photon energies larger than the energy bandgap thereof, and eachupper panel transmits solar photons with photon energies less than thelarger photovoltaic (solar) photon energies to a remaining one of thepanels lower in the hierarchy and positioned lower in the stack.Preferably the photovoltaic (solar) cells in each panel are connected inseries electrically; and the panels are connected in parallel.Alternatively it is preferred that the photovoltaic (solar) cells ineach panel are connected electrically in parallel and the panels areconnected in series. Preferably the number of solar cells on each of thepanels connected in series electrical arrangement is equal to a desiredoutput voltage of each panel divided by the operating voltage of eachthe photovoltaic (solar) cells on each panel. It is also preferred thata stack of two panels with GaAs photovoltaic (solar) cells is mounted onthe upper panel and silicon solar cells are mounted on the lower panel.

In another embodiment, the upper panel is mounted with photovoltaic(solar) cells taken from the group of GaAs, GaInP, GaAsP, amorphoussilicon, CdTe, and CdZnTe; and the lower panel is mounted withphotovoltaic (solar) cells selected from the group consisting ofcrystalline silicon, polycrystalline silicon, copper indium galliumdiselenide, germanium, gallium indium nitride, and gallium indiumarsenide nitride.

It is also preferred that a stack of three of the panels includes anupper panel, a middle panel and a lower panel arranged from top tobottom in that order; with the upper panel being mounted withphotovoltaic (solar) cells whose energy bandgap is larger than 1.7electron volts, with the lower panel being mounted with photovoltaic(solar) cells with an energy bandgap is less than 1.1 electron volts,and with the middle panel being mounted with photovoltaic (solar) cellswhose bandgap lies between bandgaps of the cells mounted on the upperpanel and lower panels. Preferably the photovoltaic (solar) cells ineach panel are juxtaposed and either separated by a dielectric spacer;or butted together.

In accordance with another aspect of this invention, a photovoltaic(solar) energy conversion device includes at least two panels arrangedfor direction of photovoltaic (solar) energy to be incident upon anuppermost panel in the stack with each of the panels containing an arrayof photovoltaic (solar) cells having a different energy bandgap. Thepanels are arranged in a vertical stack with the panels with higherenergy bandgap photovoltaic (solar) cells situated above the panels withlower energy bandgap photovoltaic (solar) cells; and with each panelbeing adapted to absorb a fraction of sunlight with photovoltaic (solar)photon energies larger than its energy bandgap and being adapted totransmit photovoltaic (solar) photons with energies less than its energybandgap to the panels lower in the stack.

Preferably the photovoltaic (solar) cells in each panel are connectedelectrically in series and the panels are connected in parallel; or thephotovoltaic (solar) cells in each panel are connected electrically inparallel and the panels are connected in series. Preferably, the numberof photovoltaic (solar) cells on each panel connected in parallelelectrical arrangement is equal to a desired output current of eachpanel divided by operating current of each the solar cells on eachpanel.

The transparency of the upper panels and solid lower-most panel providesthat some fraction of light transmitted through upper-lying panels maybe reflected back upward by lower-lying panels to result in additionalpower output and lower losses. The benefit of such upward reflectiondepends on the amount of light which penetrates each panel and thereflection properties of the lower-lying panels.

It is preferred that a stack of two panels with GaAs photovoltaic(solar) cells is mounted on the upper panel and silicon solar cells aremounted on the lower the panel. Alternatively, an upper panel is mountedwith solar cells taken from the group of GaAs, GaInP, GaAsP, amorphoussilicon, CdTe. and CdZnTe; and the lower the panel is mounted withphotovoltaic (solar) cells taken from the group of crystalline silicon,polycrystalline silicon, copper indium gallium diselenide, germanium,gallium indium nitride, or gallium indium arsenide nitride.

In accordance with still another aspect of this invention, three of thepanels of photovoltaic (solar) cells in a stack include an upper panel,a middle panel and a lower panel arranged in that order from top tobottom. The upper panel is mounted with solar cells whose energy bandgapis larger than about 1.7 electron volts. The lower panel is mounted withsolar cells whose energy bandgap is less than about 1.1 electron volts.The middle panel is mounted with solar cells with a bandgap which liesbetween the bandgaps of the cells mounted on the upper panel and thelower panel. Preferably, a group of the photovoltaic (solar) cells isdesignated as a unit and a protective by-pass diode is included witheach unit. Preferably the photovoltaic (solar) cells in each panel areseparated by a dielectric spacer; or the photovoltaic (solar) cells ineach panel are butted together.

The invention and objects and features thereof will be more readilyapparent from the following detailed description and appended claimswhen taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Tandem Energy Conversion (TEC) device in accordance withthis invention consisting of two stacked panels of solar cells.

FIG. 2 shows a Tandem Energy Conversion (TEC) device in accordance withthis invention consisting of three stacked panels of solar cells.

FIG. 3A shows a Tandem Energy Conversion (TEC) solar cell device inaccordance with this invention with upper and lower panels of solarcells in which the solar cells on each panel are connected in electricalseries and with the panels connected in electrical parallel. FIG. 3B isan electrical schematic circuit diagram of the TEC solar cell deviceshown in FIG. 3A.

FIG. 4A shows an alternative arrangement of a Tandem Energy Conversion(TEC) solar cell device in accordance with this invention with upper andlower panels of solar cells in which the solar cells on each panel areconnected in electrical parallel instead of electrical series and withthe panels connected in series. The solar cells on each panel are spacedapart from each other by dielectric spacers. FIG. 4B is an electricalcircuit diagram of the TEC solar device shown in FIG. 4A.

FIG. 4C is a modification of the TEC solar cell device of FIG. 4A withthe solar cells of each panel butted up against each other with thepanels connected in series. FIG. 4D is an electrical circuit diagram ofthe TEC solar device shown in FIG. 4C, which is identical to FIG. 4B,since the electrical connections have not been changed.

FIG. 4E is a modification of the TEC solar cell device of FIG. 4C, withthe solar cells of each panel butted up against each other and with thepanels connected in parallel.

FIG. 4F shows a circuit diagram for the TEC solar cell device of FIG. 4Eshowing the parallel electrical connections of the panels.

FIG. 5A shows a tandem energy conversion (TEC) solar cell device inaccordance with this invention in which the solar cells of alternate p/nand n/p variety are connected in electrical series. FIG. 5B is a circuitdiagram of the TEC solar cell device of FIG. 5A.

The detailed description which follows explains the preferredembodiments of the invention, together with advantages and features withreference to the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION TandemSolar Panels

FIG. 1 shows a Tandem Energy Conversion (TEC) solar cell device 10consisting of a stack ST1 of a tandem arrangement of two solar cellpanels (as examples of panels of photovoltaic cells) including an uppersolar cell panel 11 with three upper solar cells 12 and a lower solarcell panel 15 with lower solar cells 14. As shown, the lower solar cellpanel 15 lies below the upper solar cell panel 11, but the structure maybe inverted if electromagnetic energy is supplied from therebelow. Theupper solar cell panel 11 comprises a transparent top substrate 13having a top surface on which a Transparent Conducting Oxide (TCO) pad18 is mounted. The TCO pad 18 has a top surface on which the three,wide, upper solar cells 12, which are diodes are juxtaposed,side-by-side, in parallel with narrow spaces therebetween.

Only three upper solar cells 12 are shown for convenience ofillustration, but many more are likely to be employed. The upper solarcells 12 generate direct current electricity with electricallyconductive electrodes, i.e. anodes and cathodes on opposite surfacesthereof adapted for connection in an electric circuit. There is an upperelectrode on the top surface of each of the upper solar cells 12 andthere is a lower electrode on the bottom surface of each of the uppersolar cells 12.

As is the case for all diodes, as is well understood by those skilled inthe art, the upper and lower electrodes of the solar cells (diodes) haveopposite polarities. For example all of the upper electrodes may becathodes, which have negative polarities, and in that case, all of thelower electrodes will be anodes which have positive polarities or thereverse depending upon how they are to be connected in a circuit. Thewidths of the narrow spaces between the upper solar cells 12 areminimized so that little if any light passes between them. The TCO layer18 is composed of a material which allows light which is not absorbed bythe set of several, upper solar cells 12 to be transmitted therethrough.Then the light which was not absorbed by the upper solar cells 12 willbe transmitted in turn through the transparent top substrate 13 and downonto the lower solar cell panel 15.

In particular, the transparent TCO layer 18 may be composed of anelectrically conductive material such as indium-tin oxide, tin oxide,zinc oxide, and the like. The lower electrodes of the solar cells 12 arebonded to the TCO layer 18, i.e. in electrical and mechanical contacttherewith; and the TCO layer 18 can be used as a lower electrode to makean electrical connection at a location thereon such as the exposed end 9of the TCO layer 18.

The lower solar cell panel 15 includes a bottom substrate 16, having atop surface with several lower solar cells 14 (shown as only four forconvenience of illustration) formed juxtaposed in parallel, with narrowspaces therebetween on the top surface of the bottom conductivesubstrate 16. Only four lower solar cells 14 are shown for convenienceof illustration, but many more are likely to be employed. The four lowersolar cells 14, which are narrower than the upper solar cells 12, arealso diodes with upper and lower electrodes having opposite polaritieson the top surface and the bottom surfaces thereof.

For example, as described above for the upper solar cell panel 11, it ispreferred that all of the upper electrodes may have negative polaritiesand all of the lower electrodes may have positive polarities or thereverse and that may be altered, depending upon how they are to beconnected in a circuit as will be well understood by those skilled inthe art. The conductive, bottom substrate 16 does not need to betransparent and can be composed of an electrically conductive materialsuch as metal which can be used as a lower electrode to make anelectrical connection at a location thereon such as the exposed end 19of the bottom substrate 16.

Radiant energy 17, which may be light, sunlight, or other radiant energyfrom a solar or other energy source is shown directed downwardly to beincident upon the stack ST1 including the top surfaces of the uppersolar cells 12. The majority of the portion of the solar spectrum withphoton energies above the bandgap of the upper solar cells 12 isabsorbed thereby and is converted thereby into electrical energy.

The portion of the solar spectrum with photon energies below the bandgapof the upper solar cells 12 is transmitted down through the upper solarcells 12, the TCO layer 18 and the transparent top substrate 13 downonto the top surfaces of the lower solar cell panel 15 and onto thelower solar cells 14 mounted thereon where the portion of the solarspectrum with energies above the bandgap of the lower cells 14 isconverted into electrical energy. The spaces between the upper solarcells 12 and the lower solar cells 14 have minimum widths so that amaximum amount of sunlight is collected for the minimum cell area;otherwise the spaces between the upper and lower solar cells 12 and 14would constitute efficiency losses.

Only three upper solar cells 12 are shown on the upper solar cell panel11 and only four lower solar cells 14 are shown on the lower solar cellpanel 15 for convenience of illustration. In practice, the number ofcells on each of the solar cell panels 11 and 15 is determined by thedesired voltage output of the panel, divided by the output voltage ofthe individual cells thereon.

Multiple Tandem Solar Panels

FIG. 2 shows a Tandem Energy Conversion (TEC) solar cell device 20consisting of another stack ST2 of multiple (three) solar cell panels11, 21 and 23 with three sets of solar cells 12, 22, and 24, which arediodes, mounted thereon, respectively. The solar cells 12, 22, and 24are diodes which have both upper electrodes formed on the top surfacesthereof and lower electrodes formed on the bottom surfaces thereof, aswill be well understood by those skilled in the art. The upperelectrodes and the lower electrodes would usually have oppositepolarities. For example all of the upper electrodes may have negativepolarities and all of the lower electrodes may have positive polaritiesor the reverse but are consistent.

The upper panel 11 and the middle panel 21 are composed of transparenttop substrate 13 and transparent middle substrate 13′ on the bottom ofthe top and middle solar cell panels 11 and 21 with an upper TCO layer18 and an intermediate TCO layer 18′, respectively, formed on the topsurfaces thereof. The upper TCO layer 18 can be used as a lowerelectrode to make an electrical connection at a location thereon such asthe exposed end 9 thereof. Similarly, the intermediate TCO layer 18′ canbe used as a lower electrode to make an electrical connection at alocation thereon such as the exposed end 9′ thereof.

As in FIG. 1 there are a few upper solar cells 12, e.g. three, mountedon the top surface of the TCO layer 18 of the upper panel 11 with theirlower electrodes bonded to the intermediate TCO layer 18 i.e. inelectrical and mechanical contact therewith. There are more, usuallysmaller intermediate, middle, solar cells 22, shown as only four cellsfor convenience of illustration, formed on the top surface of theintermediate TCO layer 18′ of the middle panel 21 of middle solar cells22.

Even more lower solar cells 24, shown as only seven (for convenience ofillustration), are formed on the top surface of the conductive substrate16′ of the bottom panel 23 of solar cells 24. As with FIG. 1, the bottomsubstrate 16′ does need not be transparent and can be composed of anelectrically conductive material such as metal which can be used as alower electrode to make an electrical connection at a location thereonsuch as the exposed end 19′ of the bottom substrate 16′. Sunlight,light, or other radiant energy 17 is incident on the upper solar cells12 and the exposed top surfaces of the TCO layer 18 of the upper panel11, as in FIG. 1.

The solar cells shown in FIGS. 1 and 2 decrease in size on lower panelsin order to result in the same net voltage output from each panel, arequirement for connecting the panels in parallel. This requirement ondevice size is not necessary if panels are connected in series where thecurrent output must be the same instead of the voltage output.

The majority of the portion of the solar spectrum in the light, sunlightor other radiant energy source 17 with photon energies higher than thebandgap of the upper solar cells 12 of the upper panel 11 is absorbed bythe upper solar cells 12 and is converted into electrical energy.However, the portion of the solar spectrum with photon energies belowthe bandgap of the upper cells 12 is transmitted through the upper solarcells 12, the TCO layer 18 and the transparent top substrate 13 down topanel 21 where the portion of the solar spectrum with energies above thebandgap of the middle solar cells 22 is converted into electricalenergy.

The portion of the solar spectrum above the bandgap of the middle solarcells 22 in the middle panel 21 is absorbed by the solar cells 22. Theremainder of the energy, i.e. the portion of the solar spectrum withphoton energies below the bandgap of the middle solar cells 22 istransmitted through the intermediate TCO layer 18′ and the transparentmiddle substrate 13′ down towards the bottom cells 24 on the bottompanel 23, where solar energy absorbed thereby is converted intoelectrical energy.

The solar cells on each panel do not need to be all the same size. Whatis required is to connect the cells on each panel in either electricalseries, parallel, or combinations of series and parallel so that thedesired output is obtained. It is also possible to use different bandgapsolar cells on a single panel if a particular advantage is obtained bydoing so; however, generally it is desirable to mount solar cells of thesame bandgap on each individual panel.

Solar Cell Materials with Different Bandgaps

A range of materials is available for use as the solar cells for eachpanel. For example, the upper panel 11 in FIG. 1 can be mounted withcells whose bandgap equals or exceeds 1.4 electron-volts (eV) such asamorphous Silicon (aSi), Gallium Arsenide (GaAs), Cadmium Zinc Telluride(CdZnTe), Gallium Indium Phosphide (GalnP), or Cadmium Telluride (CdTe),while the lower panel can be mounted with cells whose bandgaps are 1.1eV or less such as silicon (Si), germanium (Ge), Indium-Gallium Arsenide(InGaAs), Gallium Indium Nitride (GaInN), Gallium Indium ArsenideNitride (GaInAsN), or Copper-Indium-Gallium-diSelenide (CIGS). Othermaterials can be employed having bandgaps which are within the preferredranges.

For the upper cells 12 of FIG. 2, materials with yet higher bandgapssuch as equal to or greater than 1.7 eV are preferred such as aSi,Gallium-Indium Phosphide (GalnP), aluminum gallium phosphide (AlGaP),Gallium Aluminum Arsenide (GaAlAs), and Gallium Arsenide Phosphide(GaAsP), where the bandgaps can be tailored by the compositions of thesemiconductor alloys.

For the middle panel 21, the middle solar cells 22 can be made withsemiconductors whose bandgaps lie below 1.7 eV but above 1.0 eV such asSilicon (Si), Gallium Arsenide (GaAs), Gallium Indium Arsenide (GaInAs),CIGS, and Gallium Indium Arsenide Nitride (GaInAsN).

For the lower panel 23, solar cells can be made with semiconductors withbandgaps below 1.1 eV such as GaInAs, silicon-germanium (SiGe), Ge,gallium antimonide (GaSb), and alloys of these materials, where thebandgap is determined by the alloy composition.

As in FIG. 1, the number of cells 12, 22, and 24 on panels 11, 21, and23 are determined by the desired voltage output of the panels and thevoltage outputs of the individual cells.

TEC Solar Cell Panels with Solar Cells Connected in Series on SeparateTCO Pads

FIG. 3A shows another embodiment of a Tandem Energy Conversion (TEC)solar cell device 30 comprising a stack ST3 of two solar cell panelsconsisting of an upper panel 25 and a lower panel 31, in which theseparate solar cells 27 on the upper panel 25 are connected in a firstset of electrical series connections. The separate solar cells 33 on thelower panel 31 are connected in a second set of electrical seriesconnections. As described below, the panels 25 and 31 are connected inelectrical parallel between terminals T1 and T2 as shown in FIGS. 3A and3B. FIG. 3B is an electrical schematic circuit diagram of the TEC solarcell device 30 shown in FIG. 3A.

In FIG. 3A, the upper panel 25 contains two separate solar cells 27A and27B which are diodes with bottom electrodes and top electrodes andconnected in series by an interconnect 28, as described in more detailbelow. Each of the separate solar cell diodes 27A and 27B is formeddirectly on the top surface of a respective one of two separateelectrical conductor Transparent Conducting Oxide (TCO) pads 26A and 26Bwhich are optically transparent. In turn the two separate TCO pads 26Aand 26B are shown to be formed side by side directly on and inmechanical contact with the top surface of a first, upper, transparentdielectric substrate 37. The TCO pads 26A and 26B are used as electrodesto make electrical connections from the bottom electrode of therespective one of the separate solar cells 27A and 27B to the nextelement in the circuit. An electrically conductive interconnect 28connects the TCO pad 26A on the left to the top electrode of the righthand solar cell 27B so that the two juxtaposed and separate solar celldiodes 27A and 27B on the upper panel 25 are connected in electricalseries. In particular, the bottom electrode of the left hand solar celldiode 27A is connected through direct electrical and mechanical contactwith the top surface of the left hand TCO pad 26A which is connectedelectrically and mechanically to the interconnect 28 which in turn isconnected electrically and mechanically to the top electrode of theright hand solar cell diode 27B which is juxtaposed with the solar celldiode 27A.

In short, on the upper panel 25, the bottom electrode of the left handsolar cell diode 27A is connected electrically to the top electrode ofthe adjacent, right hand, solar cell diode 27B through the left hand TCOpad 26A which connects electrically to the interconnect 28. Thus, thevoltages of the two solar cell diodes 27A and 27B, which are connectedin series, add together. The terminal T1 is connected electrically byelectrical line 34 to a connector A1, which is connected by directelectrical and mechanical contact to the top electrode of the left handsolar cell diode 27A. The bottom electrode of the right hand solar celldiode 27B is connected by direct electrical and mechanical contact withthe top surface of the right hand TCO pad 26B which is connected bydirect electrical and mechanical contact to the connector B1 which inturn is connected electrically by electrical line 36 to the rightexternal terminal T2.

In FIG. 3A, the lower panel 31 contains a set of four separate solarcells 33A, 33B, 33C and 33D, each of which is formed on the top surfaceof a respective one of a set of four separate TCO pads 32A, 32B, 32C,and 32D. Each of those four separate TCO pads 32A, 32B, 32C, and 32D, inturn, is shown to be formed directly on and in mechanical contact withthe top surface of a bottom substrate 38. The separate solar cells 33A,33B, 33C and 33D are connected in electrical series by the TCO pads 32A,32B, 32C, and 32D and a set of three electrically conductiveinterconnects 35A, 35B, and 35C in like manner to the connectionsdescribed above on the upper panel 25. In the lower panel 31, the bottomof each of the separate cells 33A, 33B, and 33C from left to right isconnected, in electrical series, respectively to the top of the adjacentsolar cell 33B, 33C and 33D, juxtaposed therewith, on the right by thecorresponding TCO pad 32A, 32B, or 32C and the correspondingelectrically conductive interconnect 35A, 35B, or 35C. Thus, thevoltages of the separate solar cells 33A, 33B, 33C, and 33D, which areconnected in electrical series, also add together.

The left hand electrodes A1 and A2 of panels 25 and 31 are shownconnected together electrically by electrical line 34 which connectsboth of them electrically to terminal T1. The right hand electrodes B1and B2 of panels 25 and 31 are connected electrically together and toterminal T2 by line 36, which connects the panels 25 and 31 inelectrical parallel. That makes the two panel TEC solar cell device 30 atwo terminal device, as the panels 25 and 31 are connected together inelectrical parallel. The voltages of each panel are made the same. Thisis carried out by making the number of solar cells connected inelectrical series equal to the desired voltage output divided by thevoltage output of the solar cells on each panel.

For example, if the desired voltage output is 16 volts, if the solarcell diodes 27A/27B on panel 25 have an output of 1 volt, then sixteensolar cells are connected in electrical series by interconnects 28,while if the solar cells 33 on the lower panel 31 have an output of 0.5volt, then thirty-two cells 33 are connected in electrical series. Solarcells diodes 27A/27B, for example, might be GaAs with a 1 volt outputand the solar cells 33 might be Si with a 0.5 volt output.

Three or more panel energy conversion devices such as those shown inFIG. 2 are constructed by the same principle in the sense that thenumber of cells connected in series is the total desired output voltagedivided by the output voltages of the solar cells on each panel. A groupof cells connected in series on each panel to create the desired totaloutput voltage can be called a “unit” for the purposes of thisinvention. Each unit has the same voltage output and panels may containmultiple units. Units can be connected in parallel on each panel toobtain more current output.

As shown in the electrical schematic circuit diagram of FIG. 3B, the TECsolar cell device 30 may also incorporate an upper by-pass diode D1 forthe upper panel of solar cells 27A/27B and a lower by-pass diode D2 forthe lower panel of solar cells 33A/33B/33C/33D to protect the TEC solarcell device 30 in case a group of cells becomes shaded while the othersremain exposed to sunlight.

Solar Cell Panels Connected in Series with Solar Cells on a PanelSeparated by Dielectric Spacers

FIG. 4A shows an alternative arrangement of a TEC solar cell device 40Acomprising a stack ST4 of an upper panel 41 and a lower panel 51including a set of the separate, upper solar cells 43 in the upper panel41 and a set of separate lower solar cells 53 in the lower panel 51which solar cells in each panel are connected electrically in parallelinstead of being connected electrically in series. On the other hand, asshown in FIG. 4A, the upper panel 41 and the lower panel 40 areconnected in electrical series by an electrical wire line 58. Theterminal T3 is connected by electrical wire line 54 to a connector A3,which is connected by direct electrical and mechanical contact to thetop electrode of the left hand solar cell 43 in the upper panel 41. Thebottom electrodes of all of the solar cells 53 are all connected bydirect electrical and mechanical contact with the top surface of a lowerconductive substrate 15, which is connected by direct electrical andmechanical contact to the connector A6, which in turn is connectedelectrically by electrical line 59 to terminal T6. However, as explainedabove, the upper panel 41 and the lower panel 40 are connected inelectrical series.

FIG. 4B is an electrical schematic circuit diagram of the TEC solardevice 40A shown in FIG. 4A. The upper panel 41 and the lower panel 51can be connected in electrical series. In this case, it is necessary tomake the currents supplied by each panel the same. The upper panel 41and the lower panel 51 are connected in series rather than in parallelin the finished energy conversion device and the voltage outputs of thetwo panels, i.e. upper panel 41 and the lower panel 51, add together.

In FIG. 4A, the upper panel 41 includes separate, upper solar cells 43(which are diodes) separated by dielectric spacers 47 and the lowerpanel 51 includes separate lower solar cells 53 (which are also diodes)separated by dielectric spacers 57. The upper solar cells 43 of theupper panel 41 have their lower electrodes mounted in direct electricaland mechanical contact with a TCO layer 39, which is formed on the topsurface of a top transparent substrate 13, as in FIG. 1. Thus, sunlightof energy less than the bandgap of the upper solar cells 43 will betransmitted to the lower panel 51 and the lower solar cells 53. On theupper panel 41, the TCO conducting layer 39 provides electricalconnections to the lower electrodes on the back sides of the upper solarcells 43, connecting them together.

Similarly, on the lower panel 51 a conductive substrate 15 on which thelower solar cells 53 are mounted connects the lower electrodes on theback sides of the lower solar cells 53 together electrically. The upperelectrodes on the front sides of upper solar cells 43 and the upperelectrodes on the front sides of the lower solar cells 53 are connectedelectrically in parallel by upper connectors 45 and lower connectors 55respectively as indicated respectively in FIG. 4B. The upper electrodesof the solar cells 43 and 53 are shown with dual connections to theupper electrodes in view of the mechanical spacing between theconnections points to the upper electrodes at each end of the A3 and theupper connectors 45 and the lower connectors 55.

Solar Cell Panels Connected in Series Electrically with Cells in a PanelButted Together

FIGS. 4C and 4D show an alternative arrangement to FIGS. 4A and 4B inwhich the dielectric spacers 47 and 57 have been omitted so that thesolar cells on the panels 41 and 51 are butted together, but otherwisethe structures and the circuits are the same. As shown in FIG. 4C, theoptional dielectric spacers 47 and 57 in the stack ST4 in FIG. 4A whichisolate individual cells 43 and 53 respectively from adjacent cells havebeen omitted from the embodiment of the invention shown in FIG. 4C. Ascontrasted to FIG. 4A in FIG. 4C the stack ST5 comprises an alternativeparallel connection of the solar cells 43 and 53 which are respectivelybutted up against each other. The circuit diagram shown in FIG. 4D isidentical to FIG. 4B, since the electrical connections have not beenchanged.

FIG. 4C is a modification of the TEC solar cell device of FIG. 4A withthe separate, solar cells of each panel butted up against each otherwithout being separated by the dielectric spacers 47 of FIG. 4A, butbeing the same as FIG. 4A in that the panels are shown connected inelectrical parallel. In the upper panel 41 of FIGS. 4A and 4C, theelectrodes 45 on the top and TCO conducting layer 39 on the bottom areprovided for connecting the upper solar cells 43 together in the upperpanel 41 in electrical parallel between the terminals T3 and T4. In thelower panel 51, the electrodes 55 on the top and the TCO conductinglayer 15 on the bottom of the lower panel 51 are provided for connectingthe lower solar cells 53 in the lower panel together in electricalparallel between the output terminals T5 and T6 on the lower conductivesubstrate 15.

In the lower panel 51, the lower connectors 55 on the top of the lowersolar cells 53 and the conductive substrate 15 on the bottom of thelower panel 51 are provided for connecting the lower solar cells 53 inthe lower panel together in parallel between the connector A5 on theupper left solar cell 53 and the connector A6 on the top surface of theright hand end of the lower substrate 16.

The connectors A3 and A4 of the upper panel 41 and the connectors A5 andA6 of the lower panel 51 are provided for connecting the upper panel 41and the lower panel 51 together in series or in parallel. For example,as shown in FIGS. 4A, 4B, 4C and 4D the connectors A4 and A5 can beconnected together to place the panels in electrical series, with theoutput voltage of the solar cell device 40A appearing across theconnectors A3 and A6. Those combinations determine which are theexternally connected, i.e. output, electrodes of the energy conversiondevice.

Each panel must have the same current output in order for the panels tobe connected in electrical series. The number of cells which need to beconnected in parallel on each panel is the total output current dividedby the current output of each cell. For example, if each panel isintended to provide an output current of six (6) amperes, twelve (12)cells are connected in parallel if each cell outputs 0.5 amperes, andfifteen (15) cells are connected in parallel if each cell outputs 0.4amperes.

Solar Cell Panels Connected in Parallel with Cells in a Panel ButtedTogether

Alternatively, as will be well understood by those skilled in the art,FIGS. 4E and 4F, are modifications of FIGS. 4C and 4D. FIG. 4E is amodification of the TEC solar cell device of FIG. 4C, with the separate,solar cells 43 and 53 of each panel 41 and 40 butted up against eachother, respectively, and with the panels 41 and 40 connected inelectrical parallel by wires 58A and 58B. FIGS. 4E and 4F show theconnectors A3 and A5 connected together by line 58A and the connectorsA4 and A6 connected together by line 58B to connect the panels 41 and 51electrically in parallel.

Each such group of parallel-connected solar cells which outputs adesired current can be considered a unit. Panels may contain multipleunits, each of which has the same current output. Units on each panelcan be connected in series to add voltages at a substantially constantcurrent, or further connected in parallel to obtain more current at asubstantially constant voltage. Each unit may also incorporate a by-passdiode to protect the unit in case a group of cells becomes shaded fromsunlight while the others remain exposed to sunlight.

Solar Cell Panel with Adjacent Solar Cells Physically Inverted andConnected in Series

FIG. 5A shows a tandem energy conversion (TEC) solar cell device inaccordance with this invention in which the solar cells of alternate p/nand n/p variety are connected in electrical series. FIG. 5B shows thecircuit diagram of the TEC solar cell device shown in FIG. 5A. FIG. 5Ashows an alternating series connection of a set of solar cells 63A, 64A,63B, 64B, 63C and 64D on a panel 60 including a first set of p-n solarcells 63A, 63B, and 63C which are diodes and a second set of upsidedown, i.e. physically inverted, n-p solar cells 64A, 64B, and 64C whichare also diodes formed on a set of three TCO pads 62A, 62B and 62C.

The solar cells 63A, 64A, 63B, 64B, 63C and 64D are juxtaposed on thetop surface of a substrate 61 which will be transparent in a case inwhich there are other solar cell panels therebelow (not shown forconvenience of illustration.) The polarities of the successivejuxtaposed first set of solar cells 63A, 63B, 63C and the second set ofsolar cells 64A, 64B, 64C alternate from p/n junctions, to n/pjunctions, to p/n junctions, to n/p junctions, etc., because thepolarities thereof are reversed from top to bottom. In other words panel60 consists of several solar cells 63A, 63B, and 63C having p-type topsurfaces and n-type bases which are juxtaposed with solar cells 64A,64B, 64C that have n-type top surfaces and p-type bases, etc.

The conductive TCO pads 62A, 62B, and 62C add the voltages of adjacentcells and connecting electrodes 65A and 65B add the voltages of adjacentpairs of cells 64A and 63B located on TCO pads 62A and 62B and adjacentpairs of cells 64B and 63C located on TCO pads 62B and 62C.

External electrodes 66 and 67, which are provided for externalconnection to the panel 60, are connected to cell 63A and cell 64Crespectively. The anode of cell 63A is connected to the externalelectrode 66 and the cathode of cell 63A is connected and bonded to theTCO layer 62A. The anode of cell 64A is connected and bonded to the TCOlayer 62A and the cathode of cell 64A is connected to the connectingelectrodes 65A. In turn, the anode of cell 63B is connected to theconnecting electrode 65A and the cathode of cell 63B is connected to andbonded to the TCO layer 62B. The anode of cell 64B is connected andbonded to the TCO layer 62B and the cathode of cell 64B is connected tothe connecting electrode 65B. In turn, the anode of cell 63C isconnected to the connecting electrode 65B and the cathode of cell 63C isconnected and bonded to the TCO layer 62C. The anode of cell 64C isconnected and bonded to the TCO layer 62C and the cathode of cell 64B isconnected to the external electrode 67. FIG. 5B shows the connections ofthe devices between the external electrodes 66 and 67 as described indetail above.

Each panel in a stack of panels, such as panel 60 in FIG. 5A, contains anumber of cells connected in series equal to the desired total outputvoltage of the panel divided by the output voltage of each cell.

For example, the panel 60 may contain groups of sixteen cells outputtingone volt at a substantially constant current and a second panel maycontain groups of thirty-two cells each outputting 0.5 volt at asubstantially constant current. A group of such cells connected inseries to obtain a desired voltage output represents a unit. Panels maycontain multiple units connected in series for higher voltage output orparallel for higher current output or a combination of series andparallel. By-pass diodes can be incorporated with each unit or group ofunits to protect the unit against partial sunlight shading. Bypassdiodes can also be used with groups of cells within a unit to protectthe group against partial sunlight shading.

Tandem Energy Conversion (TEC) devices, consisting of panels connectedin parallel to each other which then have the same voltage output, havean advantage over panels connected in series that require equal currentoutputs. As the incident solar spectrum changes over the day and overthe year, the current outputs of a set of series connected panelschanges linearly with changes in the spectrum, while the voltage outputsof parallel-connected panels change logarithmically with spectrumchanges, making the parallel panel connection less sensitive to suchsolar spectrum changes than the series panel connection.

It will be clear to one skilled in the art that combinations of seriesand parallel connections both of solar cells on each panel and thepanels themselves can be made to effect the desired result of voltageand current output.

A significant feature of the invention is the placing of panels of solarcells in optical series such that each panel absorbs a fraction of theincident solar spectrum and transmits the rest, then connecting thepanels in electrical series (where each panel must output the samecurrent) to add their voltage outputs or in electrical parallel (whereeach panel must output the same voltage output) to add their currentoutputs.

An added benefit of fabricating each panel separately and combining themat the finish of the fabrication process is that each solar cell can bemanufactured separately and therefore using optimized processes, takinginto account that each material may need different process conditionssuch as temperature, thickness, type of surface coatings if any, and soforth. The optimized solar cell panels are then combined to realize thehigh performance/efficiency of the tandem approach.

While the invention foregoing description describes exemplary specificembodiments of the present invention in terms, those skilled in the artwill recognize that the invention can be practiced with modificationswithin the spirit and scope of the appended claims, i.e. changes can bemade in form and detail, without departing from the spirit and scope ofthe invention. Modifications of the above disclosed apparatus andmethods which fall within the scope of the invention will be readilyapparent to those of ordinary skill in the art.

Accordingly, while the present invention has been disclosed inconnection with the above exemplary embodiments thereof, it should beunderstood that changes can be made to provide other embodiments whichmay fall within the spirit and scope of the invention and all suchchanges come within the purview of the present invention and theinvention encompasses the subject matter defined by the followingclaims.

1. A solar energy conversion device comprising: a vertical stack of atleast two panels of solar cells stacked in a hierarchy from an upperpanel to least one lower panel with each of said panels including amatching array of solar cells having an energy bandgap different fromenergy bandgaps of solar cells in other panels in said vertical stack;each of said panels in said vertical stack being arranged with a higherone of said panels having solar cells having a higher energy bandgapsituated in said hierarchy and in said vertical stack above a lower oneof said panels containing solar cells having a lower energy bandgap; atop surface of said device provided for receiving solar energy incidentupon said upper solar cell panel; said solar cells in each of saidpanels being connected electrically in parallel and said panels beingconnected electrically in series; and each of said panels absorbing afraction of sunlight with solar photon energies larger than said energybandgap thereof.
 2. The energy conversion device of claim 1 wherein thenumber of said solar cells on each of said panels which are connected inparallel electrical arrangement, for a given current through saidpanels, is equal to the total output current divided by the currentoutput of each said solar cell.
 3. The energy conversion device of claim1 comprising a stack of at least two of said panels with GaAs solarcells mounted on a said upper one of said panels and silicon solar cellsmounted on a said lower one of said panels.
 4. The energy conversiondevice of claim 2 comprising: said upper one of said panels beingmounted with solar cells taken from the group of GaAs, GaInP, GaAsP,amorphous silicon, CdTe. and CdZnTe; and a said lower one of said panelsbeing mounted with solar cells selected from the group consisting ofcrystalline silicon, polycrystalline silicon, copper indium galliumdiselenide, germanium, gallium indium nitride, and gallium indiumarsenide nitride.
 5. The energy conversion device of claim 1 comprisinga stack of at least two of said panels with GaAs solar cells mounted onsaid upper one of said panels and with silicon solar cells mounted onsaid lower one of said panels.
 6. The energy conversion device of claim1 comprising: a said upper one of said panels being mounted with solarcells taken from the group of GaAs, GaInP, GaAsP, amorphous silicon,CdTe. and CdZnTe; and a said lower one of said panels being mounted withsolar cells selected from the group consisting of crystalline silicon,polycrystalline silicon, copper indium gallium diselenide, germanium,gallium indium nitride, and gallium indium arsenide nitride.
 7. Theenergy conversion device of claim 5 comprising: a stack of three of saidpanels including an upper solar cell panel, a middle solar cell paneland a lower solar cell panel arranged from top to bottom in that order;said upper solar cell panel mounted with solar cells whose energybandgap is larger than 1.7 electron volts; said lower solar cell panelis mounted with solar cells whose energy bandgap is less than 1.1electron volts; and said middle solar cell panel is mounted with solarcells whose bandgap lies between bandgaps of said solar cells mounted onsaid upper solar cell panel and said lower solar cell panel.
 8. Theenergy conversion device of claim 1 with said solar cells in each saidpanel being separated by a dielectric spacer.
 9. The energy conversiondevice of claim 8 where a group of said solar cells is designated as aunit and a protective by-pass diode is included with each unit.
 10. Theenergy conversion device of claim 1 with said solar cells in each saidsolar cell panel being butted together.
 11. The energy conversion deviceof claim 1 with each said upper solar cell panel transmitting solarphotons with photon energies less than said larger solar photon energiesto a remaining one of said panels lower in said hierarchy and positionedlower in said stack.
 12. The energy conversion device of claim 21wherein said solar cells in each said solar cell panel being connectedelectrically in series and said panels being connected in parallel. 13.The energy conversion device of claim 11 in which a number of said solarcells on each said solar cell panel connected in parallel electricalarrangement is equal to a desired output current of each said solar cellpanel divided by operating current of each said solar cells on each saidsolar cell panel.
 14. The energy conversion device of claim 11comprising a stack of two panels with GaAs solar cells a mounted on theupper solar cell panel, and silicon solar cells mounted the lower saidsolar cell panel.
 15. The energy conversion device of claim 11comprising an upper said solar cell panel mounted with solar cells takenfrom the group of GaAs, GaInP, GaAsP, amorphous silicon, CdTe. andCdZnTe, and a lower said solar cell panel mounted with solar cells takenfrom the group of crystalline silicon, polycrystalline silicon, copperindium gallium diselenide, germanium, gallium indium nitride, or galliumindium arsenide nitride
 16. The energy conversion device of claim 11comprising: a stack of three of said panels including an upper solarcell panel, a middle solar cell panel and a lower solar cell panelarranged from top to bottom in that order; said upper solar cell panelmounted with solar cells whose energy bandgap is larger than 1.7electron volts; said lower solar cell panel mounted with solar cellswhose energy bandgap is less than 1.1 electron volts; and said middlesolar cell panel mounted with solar cells whose bandgap lies betweenbandgaps of said cells mounted on said upper solar cell panel and saidlower solar cell panel.
 17. The energy conversion device of claim 11where a group of said solar cells is designated as a unit and aprotective by-pass diode is included with each said unit.
 18. The energyconversion device of claim 11 with said solar cells in each said solarcell panel being juxtaposed and separated by a dielectric spacer. 19.The energy conversion device of claim 11 with said solar cells in eachsaid solar cell panel being juxtaposed and butted together.
 20. Theenergy conversion device of claim 16 wherein: a group of said solarcells is designated as a unit and a protective by-pass diode is includedwith each said unit; said solar cells in each said solar cell panel arejuxtaposed and either separated by a dielectric spacer or buttedtogether.